Replacement sky remote controllers

Tried Ebay?

ghetto,,  the arrow button to the left of the select button , on being pressed should allow you to go to the previous channel

https://www.saga.co.uk/magazine/technology/home-technology/film-tv/ten-thing...

The remote isn't the thing that does the channel flip it sends a code to the TV/box so it's up to the box to do the clever work.
If it's an old sky box it may not support it, not sure about newest, fairly sure the classic Sky didn't
I'd be surpried if a new box would have a remote that didn't do it.

Remote controls tend not to wear out as such in my experience, they tend to get a surprising amount of gunge inside them that stops the contacts working.
You could pick another up on ebay first perhaps but if you take the old one apart then you can clean the contacts and hopefully work as new, but the screws may be odd torx type which without tool may require a drill :)

Remotes are normally all the same, a rubber buttony sheet over a circuit board with sets of areas where if the button comes down the pad on bottom completes a simple circuit, the gunge stops the circuit completing. Which is why in the final stages they tend to work if you push "extra hard". Familiar?